The epoxy resins constitute a well known class of resin materials containing the 1, 2 - epoxide group: ##STR1##
The epoxy resins are not self-hardening, thus requiring the presence of chemical hardening agents to convert them to thermosetting resins. A well known class of hardening agents for epoxy resins is polycarboxylic acid anhydrides. While these anhydrides are effective for curing most solid epoxy resins, they are ineffectual (requiring higher curing temperatures) for most liquid epoxy resins. Consequently, catalysts are used to help promote the cure of epoxy-anhydride systems. A common class of catalysts which have been used for curing liquid epoxy-anhydride resins are tertiary amines such as those described in U.S. Pat. No. 3,052,650. Stannous salts, such as stannous octoate, have also been used for this purpose, e.g., U.S. Pat. No. 3,201,360. Various zinc salts have also been proposed such as in U.S. Pat. No. 3,364,159.
For certain uses, it may be desirable to form a one part catalyzed heat curable resin system which can maintain its viscosity at room temperature for an extended period of time and which then can subsequently be heat cured with effective hardening. As disclosed in U.S. Pat. No. 3,692,715, an epoxy resin selected from the group of polyglycidyl ethers of polyhydric phenols and polyglycidyl esters of aromatic and cycloaliphatic polycarboxylic acids can be admixed with a dicarboxylic acid anhydride in an amount sufficient to provide a specified ratio of anhydride groups per oxirane group. This resin system is catalyzed with an aliphatic carboxylic acid salt of a polyvalent metal selected from the group consisting of a titanium, zirconium, hafnium, cerium and thorium. It is noted that, in Example VIII, a diglycidyl ether of bisphenol A is admixed with tetrapropenylsuccinic anhydride and is cured with a solution of cerium neodecanoate in mineral spirits. This experiment can be genericized as using a cerium carboxylate in non-reactive organic solvent as the curing catalyst for an epoxy-anhydride resin system.
Attempts to prepare a composition similar to that disclosed in U.S. Pat. No. 3,692,715 were not successful. Applicant noticed that the cerium neodecanoate in mineral spirits was not freely and easily dissolvable in epoxy resin. Applicant used a commercial epoxy resin which is a diglycidyl ether of bisphenol A having an average epoxy equivalent of 185-192 and a greater viscosity than the material used in Example 1 of U.S. Pat. No. 3,692,715. The improperly and incompletely dispersed rare earth catalyst composition provided uneven thermal curing of the epoxy resin.
It has now been found that rare-earth carboxylates dissolved in a reactive solvent can be used as catalyst to overcome these problems.